The present invention relates to an electric cooling fan, and more particularly to a brushless DC ring motor cooling system for use in diesel power applications such as over-the-road trucks.
Diesel power applications such as over-the-road trucks, off-road equipment and agricultural equipment require a cooling system to serve a variety of cooling needs in the equipment. These systems typically contain a number of heat exchangers, a cooling fan, and in some cases a fan drive. In cases where a fan drive is not used, the fan is driven by a belt and continually rotates at a fixed ratio to engine speed. At least three sub-systems are served by the cooling fan, including the engine cooling system, the charge air system and air conditioning system. Other systems such as a transmission cooling system and hydraulic cooling system could also be served by the cooling fan.
Typical fan drives maybe implemented as on/off clutches, viscous clutches or hydraulic systems, for example. On/off clutches are usually mounted to the front of the engine block, and the clutch is belt driven by the crankshaft pulley. In some cases, the on/off clutch is mounted on the water pump, which also turns at a speed related to engine revolutions-per-minute (RPM). On/off clutches may be pneumatic, hydraulic, electric or spring engaging.
Viscous clutches are driven by the same general mechanisms as on/off clutches, except that the clutch is engaged and disengaged by varying the flow path of a viscous fluid through the clutch. Hydraulic clutches may be implemented in several ways, such as by a multiple interface clutch or a self-contained pump and motor assembly. Some hydraulic systems allow the cooling system to be remotely mounted, where belting from the crankshaft is impractical.
A cooling fan is mounted to the fan drive. Typically the fan is an axial flow, circular, plastic injection molded device. Alternatively, the fan could be constructed of a lightweight metal. The fan is located in a fan shroud which is attached to the heat exchanger adjacent to the front of the engine. The fan shroud serves as an adapter which directs the flow from the circular fan through the rectangular heat exchangers. A typical spacing between the fan and the fan shroud is about 0.5 inches to 2.0 inches per side. The large tip clearance is necessary due to the fact that the fan is engine mounted and the shroud is frame mounted, with the potential for displacement between the engine and the frame.
The cooling system can be controlled either by discrete sensors on one or more of the cooling sub-systems to turn the fan on and off, or by electronic controls received from the engine control module (ECM). Many diesel power systems currently employed in the vehicular industry are electronically controlled by an ECM, which is part of an overall communications network used to supply operational information to system components of the vehicle. The ECM may additionally be programmed to engage the fan during exhaust braking, unrelated to a cooling need, in order to draw additional horsepower from the diesel power plant to help stop the vehicle.
In most over-the-road trucks, a spring or air engaged on/off clutch is employed along with a solenoid valve, a cooling fan and a fan shroud. Electronic control is usually utilized so that the fan drive turns on and off based on a signal from the ECM. In addition, a pressure switch in the air conditioning system turns the fan on and off as required. The exhaust brake also is operable to control the operation of the fan as a braking aid.
Typical engine speeds are between 600 RPM (low idle) and 2100 RPM (rated speed). Operating engine speeds are usually between 1200 RPM and 1800 RPM. A typical fan ratio is 1.2:1, thus, operating fan speeds are usually between 1440 RPM and 2160 RPM. At the rated engine speed of 2100 RPM, the fan speed can reach 2520 RPM in such a system. In an exemplary system, the typical horsepower (Hp) for a 32-inch diameter fan ranges between about 13 Hp (at 1140 RPM) and 75 Hp (at 2520 RPM), with fan horsepower increasing cubically with fan speed. The power to drive the cooling fan comes from the engine, reducing the power to the system driven by the engine and consuming fuel.
The fan has two basic operating states. Either the fan clutch is engaged and the fan is on, or the fan clutch is disengaged and the fan is off. Fan engagements can occur in response to parameters associated with a number of sub-systems. The ECM controls engagements of the fan to keep engine coolant within an operating window, typically 182° F.-210° F. for an exemplary vehicle system. The ECM will also turn the fan on in order to keep charge air below a threshold temperature, such as 150° F. in an exemplary system. The A/C system's pressure switch is typically engaged at approximately 240 p.s.i. in an exemplary vehicle system, which will turn the fan on until the pressure falls below the set point. Fan engagements due to exhaust brake application generally occur at higher engine speeds.
The duty cycle of the fan and drive is usually between 5% and 20% on time. The on time can be broken down into a percentage of fan engagements due to sub-system or ECM control (see Table 1), and into a percentage of fan engagements at operating speeds (see Table 2). Both of these are important in analyzing the requirements of the fan, because Table 1 describes which system drives cooling system engagement and Table 2, combined with actual on time, allows calculation of energy expended and clutch life. Table 1 and Table 2 are based on fan engagements observed in an exemplary vehicle engine cooling system, and will vary somewhat for different types of vehicles, engines and cooling systems.
TABLE 1     Approximate Percentage of Engagements dueto ECM/Sub-System Control.SystemCoolantCharge AirA/CExhaust Brake%13.8—45.640.5
TABLE 2 Approximate Percentage of Engagements at   Engine Operating Speeds.RPM7009001100130015001700190021002300+%28.63.410.07.314.318.713.24.10.3Generally, engagements above 1800 RPM are in the 40.5% exhaust brake category and engagements below 1200 RPM are in the 45.6% A/C category.
Analyzing the relationships in clutch driven cooling systems between the fan speeds (which are related to engine speeds) and the type of cooling needed reveals that the power diverted to the fan is not well tailored to the power required for the type of cooling requested. One of the more problematic situations is when an engine coolant fan request is made during a low engine RPM, high torque condition. In this situation, the engine is experiencing high heat rejection and requires a high fan speed to achieve the required cooling. However, the low engine RPM during this situation would require a high belt ratio (ratio of fan speed to engine speed) to turn the fan at the necessary speed. Since the belt ratio of the fan is fixed, accommodating this condition with a high belt ratio results in overspeeding of the fan during situations where the engine speed is higher, drawing more power than is needed to achieve proper cooling in that situation. This dilemma has been a necessary shortcoming in clutch driven cooling systems, since the power provided to operate the fan comes directly from the engine itself. It would be a useful improvement in the art to provide a cooling system in which the operation of the fan is directly related to the type of cooling requested, without diverting unnecessary power from the other components of the engine. Such a cooling system, employing a novel brushless DC ring motor that provides efficient performance with an advantageous geometry, is the subject of the present invention.